Actually this ticket #11379 is fixed by the following commit. I forgot to mention it in the list of fixes in the commit message. The important bit of the patch for #11379 is the new function TcSMonad.mkShadowCt, explained by Note [Keep CDictCan shadows as CDictCan].

Let's merge this.

commit 9308c736d43b92bf8634babf565048e66e071bd8
Author: Simon Peyton Jones <simonpj@microsoft.com>
Date: Sat Jan 16 00:37:15 2016 +0000
Fix a number of subtle solver bugs
As a result of some other unrelated changes I found that
IndTypesPerf was failing, and opened Trac #11408. There's
a test in indexed-types/should-compile/T11408.
The bug was that a type like
forall t. (MT (UL t) (UR t) ~ t) => UL t -> UR t -> Int
is in fact unambiguous, but it's a bit subtle to prove
that it is unambiguous.
In investigating, Dimitrios and I found several subtle
bugs in the constraint solver, fixed by this patch
* canRewrite was missing a Derived/Derived case. This was
lost by accident in Richard's big kind-equality patch.
* Interact.interactTyVarEq would discard [D] a ~ ty if there
was a [W] a ~ ty in the inert set. But that is wrong because
the former can rewrite things that the latter cannot.
Fix: a new function eqCanDischarge
* In TcSMonad.addInertEq, the process was outright wrong for
a Given/Wanted in the (GWModel) case. We were adding a new
Derived without kicking out things that it could rewrite.
Now the code is simpler (no special GWModel case), and works
correctly.
* The special case in kickOutRewritable for [W] fsk ~ ty,
turns out not to be needed. (We emit a [D] fsk ~ ty which
will do the job.
I improved comments and documentation, esp in TcSMonad.